On the stand behind the pulley there's a pair of ridges at about the 2 o'clock position. The ridges hold the tool (3mm diameter wire with a 1cm barb at the end) laterally while you push it into a hole in the pulley You'll have to spin the fan until a hole aligns with the ridges so the tool stays in place. If you're very strong, you can lean on a piece of wood set on the belt to keep the pulley from turning as you loosen the fan. Don't get hurt!

Sixto
95 S420
91 300SE
87 300SDL
83 300SD